multifile direct compile ok, direct compile error - c

Question

There are 3 files(generator.c, generator.h and main.c).
generator.c: There is only 1 function (gen fun) which is used to generate an array to store 10 random-generate numbers in generator.c.
generator.h:Declaration of generator.c.
main.c: There is only 1 function (main fun) in main.c which is used to print the number generated previously.
If generator.c is included in main.c and I compile it directly by execute "gcc main.c". The result is ok.
But while I compile it using " gcc -c generator.h, gcc -c main.c, gcc generator.o main.o ", it reported a warning "warning: assignment makes pointer from integer without a cast" at " p = gen(arr); " sentence in main funciton. And the final result was "Segmentation fault (core dumped)". The debug information showed "Cannot access memory at address" if i try to visit the value of pointer *p(i.e. array[0]) in the while loop of main function.
//////generator.c///////
int * gen( int arr[])
{
int i = 0;
int * p = arr;
int len = 10;
srand( (unsigned)((time)(NULL)));
while (i< len)
{
*p = rand() % ( len +1) + 0;
i ++;
p++;
}
return arr;
}
//////generator.h//////
int * gen( int arr[]);
//////main.c///////
int main(void)
{
int i = 0;
int arr[10]={0};
int * p;
p = gen(arr);
while (i < 10)
{
printf("output is %d\n",*p);// Segmentation fault (core dumped)
i++;
p++;
}
return 0;
}

Answer 1

Based on the addition to your question, it appears you are confused about how to include generator.h and then to compile the code. First your generator.h should be:
//////generator.h//////
#ifndef GENERATOR_H
#define GENERATOR_H 1
int *gen (int arr[]);
#endif
(edit: added appropriate Header Guards to prevent multiple inclusion of generator.h)
Your generator.c would then be:
//////generator.c///////
#include <stdlib.h>
#include "generator.h"
int *gen (int arr[])
{
int i = 0;
int * p = arr;
int len = 10;
while (i< len)
{
*p = rand() % len + 1;
i ++;
p++;
}
return arr;
}
And finally your main.c (I called it gen.c) would be:
//////main.c///////
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "generator.h"
int main(void)
{
int i = 0;
int arr[10]={0};
int *p;
srand( (unsigned)((time)(NULL)));
p = gen(arr);
while (i < 10)
{
printf ("output is %d\n",*p);
i++;
p++;
}
return 0;
}
Compile
$ gcc -Wall -Wextra -pedantic -std=c11 -Ofast generator.c -o bin/gen gen.c
(note: I would also encourage adding -Wshadow as a normal part of your compile string as well to identify any shadowed variables)
Example Use/Output
$ ./bin/gen
output is 8
output is 1
output is 5
output is 4
output is 9
output is 5
output is 4
output is 6
output is 5
output is 6
Look things over and let me know if you have further questions.